Asynchronous Serial API fixes and refactoring, part 3
Asynchronous Serial API fixes and refactoring, part 3

Updated API documentation to better explain API operation and to explicitly
point out that any event ends the operation.

kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os